1 In a small bowl, combine the soy sauce, vinegar, sesame oil, red pepper flakes and 1/8 teaspoon of the black pepper. Mince the ginger and add it to the bowl. Set the ginger-soy sauce aside.
2 In another small bowl, combine the mayonnaise, yogurt, pickle relish and 1/8 teaspoon of the black pepper. Refrigerate the tartar sauce until serving time.
3 Cut the fish into 1 1/2-inch chunks.
4 In a medium bowl, combine the cornstarch, flour, baking powder, salt and the remaining 1/4 teaspoon black pepper. In a small bowl, beat the egg with the water. Stir the egg mixture into the dry ingredients and beat to blend.
5 In a small saucepan, warm the oil until hot but not smoking (375° on a deep-fat thermometer).
6 Meanwhile, dip the fish in the batter. When the oil is hot, add the fish in batches, without crowding, and cook until golden brown, about 3 minutes, turning the fish over once or twice as it cooks. Remove the fish chunks with a slotted spoon and drain them on paper towels. Keep the fish warm in a low oven while you cook the remaining batches.